Thumbnail selection should not resize the Viewer window

[chammoru]

Problem

When selecting an image from the thumbnail browser, the Viewer window changes size according to the selected image. This feels disruptive because simply clicking a thumbnail unexpectedly changes the whole application window layout.

This is related to #69, but this issue tracks the thumbnail-browser-specific flow.

Why this matters

• Thumbnail navigation should feel like browsing within a stable workspace.
• The user's focus is disrupted when the window jumps larger or smaller after each thumbnail click.
• It becomes harder to quickly inspect a folder containing mixed-resolution images.
• The thumbnail drawer itself may feel unstable if the main window keeps resizing around it.

Expected behavior

Clicking a thumbnail should load the selected image into the current Viewer viewport without changing the outer window size.

Recommended behavior:

1. Preserve the current Viewer window size when a thumbnail is selected.
2. Fit the selected image into the existing image area by adjusting zoom / pan.
3. Keep the thumbnail drawer position and width stable.
4. Only resize the window when the user explicitly chooses a fit-to-image / auto-resize command or setting.

Reproduction idea

1. Open Viewer with the thumbnail browser enabled.
2. Navigate to a folder with images of different resolutions.
3. Click thumbnails for images with different sizes.
4. Observe that the Viewer window grows or shrinks depending on the clicked image.

Acceptance criteria

• Selecting thumbnails no longer changes the outer Viewer window size.
• Image content is fit into the existing viewport instead.
• Thumbnail drawer layout remains stable.
• Any existing auto-resize behavior is either removed from thumbnail navigation or controlled by an explicit user setting.

[chammoru]

Fixed in 1217fa2.

Selecting a thumbnail from the browser no longer resizes the Viewer window to fit each image. Like folder navigation (#69), it now keeps the current window size and fits the chosen image into the existing viewport, so stepping through a folder of mixed-resolution images stays smooth instead of making the window jump, maximize, and flicker. The same churn was also what briefly blanked thumbnails (newly revealed rows showing their placeholder during the resize), so that is resolved too.

• MFC pane: opens with LOAD_FIT_TO_WINDOW (sets the active document's layout around the open).
• Qt pane: wraps the activation in mKeepWindowOnLoad.

Verified by driving the drawer over images ranging from 240x160 to 3000x2000: the window stays fixed on every selection, where before it grew up to a maximized 1936x1048.